Explained at Life's Little Mysteries:
On the night of July 4, professional storm chaser and photographer Mike Hollingshead caught sight of an enormous bugnado in southwestern Iowa. The air above the cornfields was so thick with bugs "it looked like it was smoking," Hollingshead told Life's Little Mysteries. He captured the strange sight on camera..I can pretty well guarantee that these are midges rather than mayflies, whose emergence swarms can be so big as to be visible on radar, but individually are larger than those seen in the video.
Joe Kieper, an entomologist who is executive director of the Virginia Museum of Natural History, says they are swarms of either mayflies or midges...
But whichever type of insect they were, they literally had a field day this summer. "If it's a flooded cornfield, that would explain why there are so many critters," Kieper said. "When you get water in a field, vegetation starts to rot and the water fills with bacteria. This is food for the insects. Because there's so much food available, when they emerge as adults, you get this huge swarm."
Addendum: Midges are important sources of food for trout in the winter:
Diamesa mendotae, a cold-hardy but delicate insect also known as a midge, [have an] unusual ability to thrive in the winter, when they serve as trout food. When most other insects are idling, with eggs and larvae hidden away from the cold, midges, armed with a sort of internal antifreeze, produce several generations of offspring. Stream anglers, skiers and others who might poke into the deeper recesses of southeastern Minnesota during winter see them flying in clouds above the water or speckling the streamside snow...
[They] can remain active down to about 6 degrees below zero... The midges are also high in calories and nutrients -- "like pecan pie" for trout, Ferrington said. But their cold-hardiness is balanced by an intolerance for warmth. An increase of as little as 1.8 degrees in the average water temperature in a stream could wipe out an entire winter reproductive cycle for them.